1. Field of the Invention
The invention relates to subterranean drill bits such as those used for subterranean mining and for drilling of subterranean oil and gas wells, and related methods.
2. Description of Related Art
In subterranean drilling such as in the exploration and production of hydrocarbons, a rotating drill bit is used to create a borehole through the subsurface formations of the earth. Although a number of subterranean drill bit designs are known in the art, such designs may be broadly classified into two areas--(1) roller cone bits and (2) fixed cutter bits. The present invention is directed principally to fixed cutter bits. The term fixed cutter drill bits as used in this document refers to subterranean drill bits in which the position of the cutting surface of the cutting elements or cutters is fixed relative to the drill bit body. Although the specific design and physical appearance of fixed cutter drill bits can vary considerably, such drill bits share a number of common features. The International Association of Drilling Contractors (IADC) recently adopted a fixed cutter bit classification system based on such common features. See, e g., W. J. Winters and N. H. Doiron, "The 1987 IADC Fixed Cutter Bit Classification System," presented at the 1987 Society of Petroleum Engineers (SPE/IADC) Drilling Conference held in New Orleans, La. on Mar. 15-18, 1987.
In recent years, fixed cutter subterranean drill bit designs using diamond materials for the cutting medium have gained widespread use in the oil and gas industry, particularly for use in subterranean formations having relatively soft to medium hardness. The cutting medium in these fixed cutter diamond drill bits typically comprises natural diamond, a polycrystalline diamond compact material, or a thermally-stable poly-crystalline diamond material. Fixed cutter diamond drill bits typically include a significant number of diamond cutters distributed over the drill bit body. Although fixed cutter diamond drill bits are relatively expensive, their superior rate of penetration (ROP) (the rate at which the drill bit drills through the subterranean earthen materials) has increased their demand and made them indispensable for some applications. Notwithstanding the popularity of fixed cutter subterranean drill bits, there has been a real concern in the industry about their susceptibility to breakage. The users of the drill bits and the drill bit manufacturers have found that, by controlling more precisely the weight-on-bit (WOB) and increasing the rotational speed (RPM), increased penetration rates can be achieved. As the RPM is increased, the drill bit effective lifetime has decreased dramatically because the cutting elements on the drill bit become damaged and occasionally are violently torn from the bit body. As the cutting elements break, the penetration rate of the bit decreases. When the penetration rate falls unacceptably low, the drill bit must be withdrawn from the borehole and replaced. The drill bit can also fail catastrophically, which also requires bit replacement. The lifetimes of the drill bits can vary considerably. It is not unknown for subterranean drill bits to catastrophically fail when they are virtually new. The cost effectiveness of subterranean drilling is directly dependent upon maintaining good penetration rates and on prolonging drill bit lifetime. Replacement of drill bits is a very expensive process given the cost of operating the drilling rigs, the time required to withdraw the drill bit from the borehole, replace it, and reinsert the drill bit, and the cost of the bits themselves.
Prior attempts to improve fixed cutter subterranean drill bit durability have been closely associated with the prevailing theories of cutting element wear and drill bit failure. During the 1970s and early 1980s, the prevailing theories of cutter wear and bit failure focused primarily on heat buildup in the cutting elements. Heat buildup was believed to cause the individual cutting elements to undergo accelerated wear. Accordingly, attempts to improve drill bit durability during this time frame focused on decreasing heat buildup on the cutting elements, for example, by improving the hydraulic design of the drill bit to better cool the cutting elements.
Another theory of cutter wear and drill bit failure prevalent during the 1970s and early 1980s involved the degree of balance inherent in the drill bit. More specifically, research efforts indicated that drill bit failure was accompanied by damage to the cutting elements whereby the diamond material was chipped or broken off of its carbide support. Given the number and positioning of the cutting elements on the bit body, this cutting element damage was believed to create unbalanced lateral or radial forces on the drill bit that forced the bit body to impact the borehole wall and further damage the drill bit. Accordingly, attempts to improve drill bit durability also included efforts to balance the drill bit so that the combined or net lateral forces on the bit during its rotation in drilling were balanced.
Various approaches were also used to strengthen the individual cutting elements, such as using beveled, domed, or high back rake cutters, using larger stud support materials for the cutters, using posts behind the cutters, and increasing the amount of diamond material on each cutting element.
Although some improvements in bit durability resulted from these efforts, a satisfactory solution to the cutter breakage problem was not found.